larry@uunet.uu.net (Larry Lippman) (04/11/91)
In article <telecom11.260.5@eecs.nwu.edu> wb8foz@mthvax.cs.miami.edu (David Lesher) writes: > But there was ANOTHER distinctive noise you could hear during ring > cycle: bridge lifters. Here's the story, as it was explained to me: Bridge lifters are nothing more than a dual-winding saturable inductor, typically a WECo 1574-type inductor. Bridge lifters are most commonly used on business lines which have an off-premises extension in a telephone answering service or other location. In such cases, both loops originate in the CO. If the combined parallel impedance of both loops is low enough, degradation of transmission can occur. The bridge lifter functions to eliminate such parallel impedance. The bridge lifter inductor is specially designed to be *very* saturable at 20 mA of DC loop current (the minimum expected value for an off-hook station). This inductor is a dual-winding device intended to be connected in series with a subscriber loop. With no DC current flow, the inserted impedance of the bridge lifter could be as high as 100,000 ohms. With 20 mA of loop current, the inserted impedance is typically less than 50 ohms. Pure DC resistance of the inductor is typically 10 ohms. Both ringing and DC supervision pass through a bridge lifter inductor with almost no attenuation in either the on-hook or off-hook state. In the case of say, a business line with a bridged connection to a telephone answering service, *both* loops are connected in series with their own inductor. Therefore, when either loop is in use, the other is "transparent" unless it goes off-hook to establish loop current flow. Some variations of bridge lifters contained resistors or diodes across the inductor windings in order to minimize effects of inductive noise resulting from power lines. Bridge lifters are also used to facilitate temporary "back-tap" connections during central office cutovers if the new CO is in another building. > Party lines were intended for conservation of wire. Thus, almost all party line subscribers were in close proximity to each other resulting in only one loop leaving the CO. Bridge lifter inductors therefore had almost no application on party lines. > Cable is capacitive. When Mr. Tip was dialing, he was breaking the > loop current with the pulser in his dial. BUT, the stub going off to > Sleeve's condo had a lot of capacitance in it, and it terminated in a > good sized ringer cap, too. (Maybe several, if Ms. Sleeve had a set in > each room.) While bridge lifter inductors do have some effect on reducing dial pulse distortion, it is minimal. This should not be surprising if one considers that the DC resistance of bridge lifter inductors is only about 10 ohms, and that the frequency domain of dial pulsing is only about 10 Hz. Thus, the bridge lifter inductor presents little insertion loss at DC or 10 Hz. > So Ma called her elves at Murray Hill, and they invented a bridge > lifter. It goes between the CO and the outgoing pair and lifts > (disconnects) Mr. Tip when Ms. Sleeve is off-hook, and vice versa. I'm > now speaking out of my hat, but I THINK it only affected things during > dialing. Otherwise, how could the other party demand surrender of the > pair for an emergency? If you consider what I have explained so far, you should realize that bridge lifter inductors have an effect any time there is loop *current* - whether it be during dialing, talking or the shunt effects of ringing. > In any case, you can HEAR the bridge lifter, in an office with ringing > sidetone, (as opposed to those that give you the switch generated > tone) as a "raspy" quality to the ring. I often notice this while > calling a doctor's office. The variation in audible ringing components *could* be the result of a resonant network created by the bridge lifter, or it could be the result of a resonant network created by the ringing detection circuit in a telephone answering service. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" VOICE: 716/688-1231 {boulder, rutgers, watmath}!ub!kitty!larry FAX: 716/741-9635 [note: ub=acsu.buffalo.edu] uunet!/ \aerion!larry